1. Field of the Invention
The present invention relates in general to detecting spark plug misfires occurring during normal operation of an internal combustion engine (ICE) and, more specifically, to a strategy to determine spark plug misfires in an ICE having multiple spark plugs per cylinder.
2. Discussion of the Prior Art
As emission standards for internal combustion engines (ICEs) become increasingly stringent, there is a need for accurate engine misfire detection and reporting during normal engine operating conditions. Present ICE control systems often include engine misfire detection systems.
Systems that monitor an ICE""s angular crankshaft velocity and crankshaft acceleration to detect engine misfires are well known in the prior art. Both measurements depend largely on engine torque produced (or not produced as expected) during an ICE""s combustion stroke within each engine cylinder. Given the velocity or acceleration information, various signature analysis and/or spectral analysis methods predict or detect engine misfires.
The prior misfire detection systems are well suited for detecting misfires in single spark plug per cylinder applications since they typically seek to detect misfires based on entire cylinder event intervals as measured by, for example, a profile ignition pulse (PIP) signal. For example, in a four-cylinder engine, the PIP signal actually indicates a piston""s approach to top dead center (also referred to as Maximum Brake Torque (MBT)) of two engine cylinders based on crankshaft position. In this example, one cylinder is approaching a power stroke and one cylinder is approaching an intake stroke. This is necessary since a four-stroke engine takes two full crankshaft rotations to complete an engine cycle. Associating unexpected crankshaft acceleration or velocity fluctuation rates with any particular cylinder can indicate potential spark plug misfires. See also generally, U.S. Pat. No. 4,899,579 to Sweppy et al, U.S. Pat. No. 5,719,330 to Carr et al., and U.S. Pat. No. 5,226,322 to Armitage.
Current technology allows an ICE to have more than one spark plug per cylinder to optimize the combustion process, thereby improving fuel economy and reducing emissions. Unfortunately, the prior art detection of ignition defects generally cannot be used. The loss of one spark plug in a cylinder having at least two spark plugs could be too small to measure. Nevertheless, the detection of the loss of one of the spark plugs in this type of ICE is still important to assure low vehicle emissions.
U.S. Pat. No. 5,872,312 to Kalweit provides a method to detect defective ignition systems in an ICE having at least two spark plugs per cylinder. This patent detects spark plug misfires by completely disabling one of the spark plugs and testing the remaining active spark plug for misfires. Unfortunately, if the active plug misfires the incomplete combustion of fuel present in the cylinder would occur. This can increase vehicle emissions and potentially damage the vehicle""s catalytic converter if the unburned fuel reaches the catalytic converter.
Thus, an improved strategy to detect spark plug misfires in ICEs having multiple spark plugs per cylinder is needed to assure low vehicle emissions and to protect the vehicle""s catalytic converter. This strategy must account for the small angular sampling differences for multiple spark plug cylinders.
Accordingly, the present invention provides a strategy to detect spark plug misfire in a multiple plug per cylinder internal combustion engine (ICE). The invention uses a controller to select one spark plug within the ICE to test, a controller to retard spark timing of a spark plug not selected for testing but within the same ICE cylinder as the spark plug selected for testing, and a spark plug misfire monitor to test the selected spark plug for misfire. The misfire monitor can measure crankshaft velocity or acceleration. A spark retard system modifies spark timing in relation to the crankshaft position needed for the piston head to reach top dead center of the cylinder in the ignition cycle. Spark retard can be a calibrated value such as 10, 30, or 50 degrees after optimum spark timing, also referred to as Maximum Brake Torque (MBT) timing, but 50 degrees is preferred because it affords a more pronounced detection signal. The optimum spark retard will be engine specific and somewhat dependent on the characteristics of the misfire monitor itself.
The present invention can also set diagnostic fault codes and initiate desired failure mode actions when a spark plug misfire is detected. These actions can include modifying spark timing and the air/fuel ratio for the cylinder with the misfiring plug to optimize emissions and performance. Halting fuel supply to the cylinder would be done under some circumstances if both spark plugs in a cylinder were misfiring.
The strategy can continue until all plugs within a cylinder are tested, then repeat the sequence until all cylinders are tested. The present invention is not limited to any particular sequence of spark plug testing so long as all spark plugs are tested within one cycle of the strategy.
The present invention provides improvement over the prior art in that the strategy does not require the complete disabling of a spark plug within the same cylinder as the tested spark plug.
Other objects of the present invention will become more apparent to persons having ordinary skill in the art to which the present invention pertains from the following description taken in conjunction with the accompanying figures.